1. Field of the Invention
The present invention relates to a method of and apparatus for measuring an energy spectrum of an X-ray consisting of polychromatic photons, and to an X-ray CT apparatus using an X-ray consisting of polychromatic photons.
2. Description of Related Art
There is known an X-ray spectrum estimation method according to which a specimen is irradiated with an X-ray to reconstruct a CT image, for example, as described in an article titled “Characteristic Features of A High-Energy X-Ray Spectrum Estimation Method Based on The Waggener Iterative Perturbation Principle” by Akira Iwasaki et al., The Official Journal of the Japanese Society for Therapeutic Radiology and Oncology, Vol. 17, No. 2, pp. 79-95, 2005.
Further, JP-T-2007-524438 describes a technique for photographing a calibration phantom which is an attenuation material, together with a specimen, by irradiating them with X-rays. The resulting photographing-information is then used for calibrating light-absorption characteristics of the subject.
However, the X-ray energy spectrum estimation method (measurement method) according to the related art only measures a quantity of X-ray passed through the attenuation material. In accordance with the attenuation amount in the measured X-ray, the method estimates rough average energy of the X-ray.
Large magnitude pulse-like or direct current X-rays are irradiated in X-ray generators put to industrially and medically practical uses. Thus, many photons are simultaneously incident on a detector. Consequently, it is substantially impossible to make a simple measurement of wave height distribution for each photon.
Further, the related art method has drawbacks that accuracy of estimation results is poor, and that the estimation results have little scientific basis. Therefore, the related art method is not a practically usable technique.
In addition, in a related art X-ray CT apparatus, spectrum of energy released from an X-ray source device are unknown, and reproduction of attenuation path information necessary for reconstructing a CT image is inaccurate. Thus, the obtained CT image that has been reconstructed becomes to have its contrast representation deviating from actual density information.
Here, it is conceivable to use a Bayesian estimation equation for estimating spectrum of energy released from an X-ray source device.
However, if the Bayesian estimation equation is merely applied to the estimation of the spectrum of energy released from the X-ray source device, the estimation equation is diverged. Thus, it is difficult to obtain a desired result. Consequently, the Bayesian estimation equation cannot appropriately be applied.
An object of the present invention is to provide an X-ray energy spectrum measurement method, an X-ray energy spectrum measurement apparatus, and an X-ray CT apparatus, in which spectrum of energy released from an X-ray source device are estimated with high precision, thereby information on a path length with respect to an X-ray irradiation can be obtained precisely, and an X-ray CT image can be reconstructed with high precision.